The invention relates to a method and a device for controlling a spark erosion process in a spark erosion machine having a spark erosion electrode and a workpiece, at least two parameters whose variation is known to influence a currently prevailing state of the spark erosion process being fed individually or as a combination thereof to a logic device in order to generate at least one output signal of this logic device which is fed to a controller for at least one of the said parameters, the parameters fed to the logic device being processed therein with the aid of rules stored therein according to the laws of fuzzy logic.
A method and a device of this type are disclosed in EP-0426870. The logic device of the relevant device receives two input values (Si and .delta.Si) of which each is a parameter of the currently prevailing state of the spark erosion process, or corresponds thereto, and it supplies two output values (.delta.T and .delta.D). The rules stored in the logic device determine how these two output values (.delta.T and .delta.d) are to be varied when the input values (Si and .delta.Si) vary individually or together, and the two output values (.delta.T and .delta.D) are then processed in a controller to form input parameters (T and D) of the spark erosion process.
It is disadvantageous in this method and the corresponding device that the input values cannot be varied as a function of one another or that one of these input values cannot be varied as a function of the other.
A method and a device of the type mentioned at the beginning are also the subject matter of European Patent Application No. 91112510.2 which, although filed before the priority date of the present patent application, was not published as publication EP-0469471 until after the priority date of the present patent application. Three input values (A, B and C) are processed in the logic device of the device according to EP-0469471, which are defined as temporally sequential values or temporally different mean values of always the same parameter of the spark erosion process (electrode position), which means that it is always values of a single parameter prevailing at different times which are compared. One of these values of the parameter can be situated in the present, but the other values are necessarily situated in the past and cannot be "retroactively" varied. Accordingly, the device of EP-0469471 has a value memory, and it cannot operate in real time with regard to all the parameters of the spark erosion process. For this reason, the rules stored in the logic device cannot determine how one of the parameters fed is to be varied when another of the parameters fed varies.
It is thus also disadvantageous in this method and the corresponding device that the input values cannot be varied as a function of one another or that one of these input values cannot be varied as a function of the other.
A method and a device for controlling a spark erosion process in a spark erosion machine having a spark erosion electrode and a workpiece, and in particular a method and a device for controlling the position of the spark erosion electrode with reference to the workpiece in a spark erosion machine are disclosed in EP-A-0333170 or U.S. Pat. No. 4864091. The actual distance between the spark erosion electrode and the workpiece is compared to a desired distance, which corresponds to the currently desired process conditions for machining the workpiece. An error signal is formed from the comparison, which is directed via low-pass filter circuit to a control circuit for moving the spark erosion electrode.
A method and a device for controlling a spark erosion process in a spark erosion machine having a spark erosion electrode and a workpiece, and in particular a method and a device for controlling the position of the spark erosion electrode with reference to the workpiece in a spark erosion machine are also disclosed in EP-A-272640 or U.S. Pat. No. 4822970. The actual distance between the spark erosion electrode and the workpiece is regularly varied by means of raising and lowering movements, and these raising and lowering movements are controlled as a function of the currently prevailing and previously found process conditions for machining the workpiece.
It is disadvantageous in these methods and devices that the influence of other parameters of the machining of the workpiece is not included in the control of the movement of the spark erosion electrode.
The above-mentioned method and the above-mentioned device according to EP-A-0333170 or U.S. Pat. No. 4864091 do permit the control response of the servo-system for controlling the actual distance between the spark erosion electrode and the workpiece to be optimized for most process conditions by matching the cut-off frequency of the low-pass filter circuit to these process conditions and keeping the AC component of the error signal at a minimum. The error signal is then input into a PID controller in order to achieve the desired dynamic response of the servo-system. The output signal of the PID controller is fed to a drive of the spark erosion electrode in order to move the latter. Since, however, it is not known to model the spark erosion process in the sense of automatic control engineering, it is necessary when designing the PID controller to proceed empirically (by trial and error), which is very complicated and does not lead reliably to an optimum result.
The above-mentioned empirical approach does not suffice for very difficult spark erosion processes, for example in the case of very large machines or, by contrast, in the process of micro-erosion. Specifically, such cases give rise to a problem of instability which is caused by the time constant of the machine being unusually large or small. The influence of many parameters then bears so strongly on the stability of the spark erosion process that account has to be taken of these parameters.
However, no technical solution which can be advocated in practice exists for feeding the known controllers with even more numerous parameters of the machining of the workpiece and even more information relating to the currently prevailing and previously found process conditions. The complexity of the spark erosion process is of such a type that it is not possible to set up or implement with a justifiable outlay any algorithm for processing all the parameters known as relevant for the spark erosion process. It has always been necessary to put up with compromise solutions in the prior art.